1. Field of the Invention
The present invention concerns a bidimensional detector of ionizing radiation as well as a manufacturing process for this detector.
The ionizing radiations which can be detected by the invention can particularly consist of X-rays, gamma photons, protons, neutrons or muons.
The detector, subject of the current invention, is capable of converting incident ionizing radiation into particles, which are themselves ionizing (electrons for example) and are easier to work with than the incident ionizing radiation.
In particular, the invention may be used in the following sectors:                instantaneous radiography of highly absorbing and/or voluminous objects,        ultra rapid cineradiography of mechanical mobiles,        positioning of patients in radiotherapy,        neutronography,        protonography,        medical and biological imagery (tomographies by emission of positrons), and        imagery by coded apertures to inspect voluminous objects with low radioactivity or suspicious parcels in a passive or quasi-passive manner.        
2. Discussion of the Background
Bidimensional detectors of ionizing radiation are already known and consist of plates made of a heavy metal such as lead, or, more precisely, of a material with a high cross-section of interaction with an incident ionizing radiation.
For example, it is common to use a metal with an atomic number (Z) greater than or equal to 73 to detect X or gamma photons and a metal with an atomic number (Z) generally less than 14 or greater than 90 to detect neutrons.
Other materials, such as Gadolinium (Z=64) can also be used to detect neutrons.
Holes are drilled in the metal plates using either chemical or electrochemical etching. The plates are electrically insulated from each other if necessary, that is, when plate thickness is equal to, or greater than, a few hundred micrometers.
The holes are filled with an ionizable gas.
A high-energy incident photon (X- or gamma-) then generates at least one electron in one of the detector plates, either by Compton effect or pair-creation effect.
The incident X or gamma photon causes the released electron to move rapidly with a kinetic energy approximately equal to that of the incident photon. The electron then rapidly ionizes some molecules of the gas contained in one of the holes which the electron reaches and, generally, passes through. Slow secondary electrons which are removed from these molecules due to the ionization of the molecules, are routed along this hole and collected by means of a bias electric field, also referred to as a drift field. The slow secondary electrons are then detected in, for example, an ionization chamber or in a proportional avalanche chamber.
Such bidimensional detectors are for example described in documents [1], [2], [3], [6], and [7] which, like the other documents referenced hereafter, are listed at the end of the present description.
The decision to use a hole-detection system can be explained by the fact that such a system offers a very high degree of spatial resolution and offers a high level of performance, subject to the holes being perfectly formed and sufficiently wide.
The holes are created by means of chemical etching. This procedure is preferable to waterjet cutting which suffers from the disadvantage of generating a frontal shock when the waterjet is turned on in the beginning of the drilling of a hole.
This frontal shock chips the material in which the holes are to be cut, which splits the material and makes it unfit for use.
However chemical etching is an expensive and slow process.
Moreover, the quantity of secondary electrons collected, and by extension, the performance levels of hole-system detectors are limited since only between 10% to 30% of the secondary electrons created in the course of each gas ionization are collected.
This can be explained by the fact that chemical etching does not create holes whose interior walls are perfectly cylindrical, that is, it cuts bottlenecks in the holes which in turn causes the electric field line to be deformed and reduces the useful diameter of the holes. Thus, the performance levels of hole-system detectors are low.